State of the art electrical feedthroughs for alumina substrates, e.g., vacuum tubes are often made through cermet "vias". A via is a homogeneous conductive alumina path through an alumina insulator, typically comprising a conventional moly-manganese coating. The vias are of defined extent in the alumina substrate. See, FIG. 1a.
In brazing a metal lead to the alumina substrate, heretofore an intermediate metallized pad (e.g., molymanganese), was required between the cermet via/alumina substrate and the metal component to be brazed thereto. This metallized pad covered not only the cermet via surface, but also extended beyond its dimensions to permit subsequent brazing to the insulating alumina surface itself. The braze to the alumina surface provided additional strength, and in fact the primary strength, while the braze to the cermet via provided electrical contact.
The resultant metal/alumina bonds were not completely satisfactory. Often the metal connectors fell off during handling operations. Sample strength was limited since it was derived solely from the area of the brazed joint located on the alumina substrate. The braze area on the cermet via contributes little or no strength to the braze joint. Typical tensile strengths were about 9,000 psi or less.
Another disadvantage of the state of the art process and resultant products resides in a relatively restricted design flexibility due to the additional bulk caused by the overlap area extending beyond that necessary solely for electrical contact with the cermet via per se. Where space is at a premium, which is often the case in electronic systems, this is a significant drawback.
Furthermore, the metallization step per se adds significant cost to the entire operation and further complicates it.
Although many methods have been employed to braze metal components to various ceramic substrates, heretofore, a solution to the problem defined above has not been suggested. A representative sampling of United States patents disclosing various aspects of prior art methodology in this area include the following: U.S. Pat. Nos. 3,402,458; 3,146,514; 3,548,345; 3,874,069; 4,278,195; and 3,203,084.